Solid polymer electrolyte material liquid composition, solid polymer fuel cell and fluoropolymer

ABSTRACT

A solid polymer electrolyte material made of a copolymer comprising a repeating unit based on a fluoromonomer A which gives a polymer having an alicyclic structure in its main chain by radical polymerization, and a repeating unit based on a fluoromonomer B of the following formula (1): 
 
CF 2 ═CF(R f ) j SO 2 X  (1) 
 
wherein j is 0 or 1, X is a fluorine atom, a chlorine atom or OM {wherein M is a hydrogen atom, an alkali metal atom or a group of NR 1 R 2 R 3 R 4  (wherein each of R 1 , R 2 , R 3  and R 4  which may be the same or different, is a hydrogen atom or a monovalent organic group)}, and R f  is a C 1-20  polyfluoroalkylene group having a straight chain or branched structure which may contain ether oxygen atoms.

This is a divisional application of U.S. application Ser. No.10/025,763, filed Dec. 26, 2001.

The present invention relates to a solid polymer electrolyte material, aliquid composition, a solid polymer fuel cell and a fluoropolymer whichcan be applied thereto.

A solid polymer fuel cell is expected to be practically used as a powersource for a vehicle such as an electric car or for a small sizecogeneration system, since high levels of cell performance can beobtained, and the weight reduction and the size reduction are easy. Witha solid polymer fuel cell which is presently being studied, theoperation temperature range is low, and its exhaust heat can hardly beutilized. Accordingly, a performance is required whereby it is possibleto obtain a high power generation efficiency and a high output densityunder such an operational condition that the utilization ratio of theanode reaction gas such as hydrogen and the utilization ratio of thecathode reaction gas such as air, are high.

Heretofore, with respect to a solid polymer fuel cell, it has beenattempted to improve the cell output by a so-called three-dimensionalmodification of the reaction site in the catalyst layer by using fineparticles of a catalyst such as a metal-carrying carbon black coatedwith an ion exchange resin of the same type as or a different type fromthe polymer electrolyte membrane, as a material constituting theelectrode catalyst layer.

However, under the above-mentioned operational conditions where thereaction rate of the cell reaction is relatively high, the amount ofwater moving together with protons which move in the polymer electrolytemembrane from an anode to a cathode, and the amount of water formed andcondensed by the electrode reaction of the cathode, will increase.Therefore, a so-called flooding phenomenon, i.e. a phenomenon whereinsuch water is not readily discharged from the cathode to the exterior,and pores for supplying the reaction gas, formed in the catalyst layerof the cathode, are clogged by such water, was likely to occur. If suchflooding occurs, supply of the cathode reaction gas to the reaction siteof the catalyst layer will be prevented, whereby the desired cell outputcan hardly constantly be obtained. Therefore, in order to improve thecell output and to obtain such an output constantly, it is necessary toimprove the water repellency and the gas diffusion property withoutlowering the ionic conductivity in the electrode catalyst layer.

Whereas, if it is intended to secure water repellency and gas diffusionproperty in the catalyst layer by reducing the ion exchange capacity(hereinafter referred to as A_(R)) of the ion exchange resin in thecatalyst layer, the water content of the ion exchange resin tends to below, whereby the ionic conductivity will decrease, and the cell outputwill decrease. Further, in such a case, the gas permeability of the ionexchange resin will also decrease, whereby the supply of the gas to besupplied to the reaction site tends to be deficient. Consequently, theconcentration overvoltage will increase, and the cell output willdecrease.

On the other hand, if it is intended to improve the ionic conductivityand the gas permeability by increasing A_(R) of the ion exchange resincontained in the catalyst layer, the water content of the ion exchangeresin will increase, whereby flooding is likely to occur, and it hasbeen difficult to obtain high cell output constantly.

Therefore, JP-A-5-36418 proposes a solid polymer fuel cell wherein afluoropolymer or the like, such as a polytetrafluoroethylene(hereinafter referred to as PTFE) atetrafluoroethylene/hexafluoropropylene copolymer or atetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer isincorporated as a water repellent agent, in the cathode catalyst layer.In this specification, “an A/B copolymer” means a copolymer comprising arepeating unit based on A and a repeating unit based on B.

Further, JP-A-7-211324 proposes a solid polymer fuel cell whereinfluorinated pitch is incorporated together with PTFE in the cathodecatalyst layer. Further, JP-A-7-192738 proposes a method wherein thecatalyst surface is fluorinated, which is used to form a cathodecatalyst layer of a solid polymer fuel cell. Still further,JP-A-5-251086 and JP-A-7-134993 propose a method of letting the waterrepellency have a gradient in the thickness direction of the electrode.

However, if a water repellent agent is incorporated to a catalyst layeras in the solid polymer fuel cell disclosed in JP-A-5-36418, there hasbeen a problem such that the electrical resistance of the cathodeincreases due to the insulating property of the water repellent agent,or the gas diffusion property of the catalyst layer is damaged due to anincrease of the thickness of the catalyst layer, whereby thepolarization characteristics of the cathode at the initial stage ofstart-up tend to be worse, and the cell output can not be improved.Further, if it is attempted to increase the cell output by reducing thecontent of the water repellent agent in the catalyst layer, the waterrepellency in the catalyst layer decreases, whereby there has been aproblem that the polarization characteristics of the electrode will bedamaged in a relatively short period of time after the start-up, andfurther, flooding is likely to occur.

Further, with the solid polymer fuel cell as disclosed in JP-A-7-211324or JP-A-7-192738, it is difficult to uniformly cover the surface of thecatalyst to be incorporated in the catalyst layer, with an ion exchangeresin, whereby there has been a problem such that adequate reaction sitecorresponding to the amount of the catalyst incorporated to the cathodecatalyst layer can not be secured, and a high cell output can not beobtained constantly. Further, the solid polymer fuel cell as disclosedin JP-A-5-251086 or JP-A-7-134993 had a problem that the productionprocess tends to be cumbersome.

And, the above-mentioned problems in securing good gas diffusionproperty and water repellency in the catalyst layer of a gas diffusionelectrode have become important also in an application of the gasdiffusion electrode to another electrochemical process such aselectrolysis of water or sodium chloride for improvement of theefficiency of the process by improving the polarization characteristics.

Further, the solid polymer fuel cell which is presently studied, has aproblem such that the operation temperature range is low at a level offrom 60 to 90° C., whereby the exhaust heat can hardly be utilized. Inan application to automobiles, a fuel cell which can be operated at atemperature higher than 100° C., is desired in order to reduce thecatalyst poisoning by carbon monoxide contained in the fuel gas or toreduce the size of the cooling system.

At present, as an electrolyte for a solid polymer fuel cell, a copolymerof CF₂═CFOCF₂CF(CF₃)O(CF₂)_(n)SO₃H (n=2 or 3) with tetrafluoroethylene,is mainly being studied. However, this resin has a softening point lowerthan 100° C., and the strength decreases at a high temperature of atleast 100° C., whereby the fuel cell can hardly be operated at a hightemperature of at least 100° C. Accordingly, as a material for a polymerelectrolyte membrane especially for a fuel cell, an ion exchange resinis desired which has a softening point of at least 100° C. and which hasa strength durable for use in a fuel cell. Further, a polymerelectrolyte is contained usually in catalyst layers of an anode and acathode, and such a polymer electrolyte is also desired to preferablyhave a softening temperature higher than the operation temperature fromthe viewpoint of the durability in a high temperature operation like themembrane material.

On the other hand, for example, a copolymer of CF₂═CFOCF₂CF₂SO₃H withtetrafluoroethylene is known to have a softening temperature higher than100° C. (ACS Symp. Ser. (1989), Vol. 395, pp 370-400). However, theproduction cost is high, and it is difficult to produce it on anindustrial scale.

The present invention has been made in view of the above problems of theprior art, and it is an object of the present invention to provide afluoropolymer excellent in the ionic conductivity, water repellency andgas permeability, and a solid polymer electrolyte material made thereof,a liquid composition containing such a solid polymer electrolytematerial and a solid polymer fuel cell containing such a solid polymerelectrolyte material as a constituting material, whereby high electricoutput can constantly be obtained. Further, it is an object of thepresent invention to provide a solid polymer electrolyte material havinga softening temperature higher than ever, in order to make it possibleto operate the solid polymer fuel cell at a temperature higher thanever.

The present inventors have conducted an extensive research to accomplishthe above objects and, as a result, have found that a fluorosulfonicpolymer having an alicyclic structure in the polymer has high ionicconductivity, and when it is used as a solid polymer electrolytematerial for an electrode catalyst layer in a solid polymer fuel cell,it is possible to improve the output of the fuel cell while securingadequate ionic conductivity in the catalyst layer. The present inventionhas been accomplished on the basis of this discovery. Further, thepresent inventors have found that the above-mentioned fluorosulfonicpolymer having an alicyclic structure has a softening point higher thanthe conventional sulfonic polymer and is a material suitable for a hightemperature operation of the solid polymer fuel cell.

Thus, the present invention provides a solid polymer electrolytematerial made of a copolymer comprising a repeating unit based on afluoromonomer A which gives a polymer having an alicyclic structure inits main chain by radical polymerization, and a repeating unit based ona fluoromonomer B of the following formula (1):CF₂═CF(R^(f))_(j)SO₂X  (1)

Here, in the formula (1), j is 0 or 1, and X is a fluorine atom, achlorine atom or a group of OM. And, M in the group of OM is a hydrogenatom, an alkali metal atom or a group of NR¹R²R³R⁴. Further, each of R¹,R², R³ and R⁴ in the group of NR¹R²R³R⁴, which may be the same ordifferent, is a hydrogen atom or a monovalent organic group, preferablya hydrogen atom or a C₁₋₄ alkyl group. Further, R^(f) is a C₁₋₂₀polyfluoroalkylene group having a straight chain or branched structurewhich may contain ether oxygen atoms.

Further, in this specification, “a solid polymer electrolyte material”includes its precursor. That is, the solid polymer electrolyte materialincludes not only an ion conductive fluoropolymer having the —SO₃M groupin its molecule when X in the —SO₂X group in the formula (1) is theabove OM, but also a fluoropolymer having in its molecule a —SO₂F groupor a —SO₂Cl group which is a precursor for the —SO₃M group. In a casewhere the solid polymer electrolyte material of the present invention isa fluoropolymer having in its molecule a —SO₂F group or a —SO₂Cl group,such a polymer may be subjected to hydrolytic treatment with e.g. anaqueous solution of a base to convert it to an ion conductivefluoropolymer having a —SO₃M group in its molecule, which is useful as asolid polymer electrolyte material.

Accordingly, when the ionic conductivity of the solid polymerelectrolyte material of the present invention is discussed in thefollowing description, if the obtainable solid polymer electrolytematerial is a fluoropolymer having in its molecule a —SO₂F group or a—SO₂Cl group by X in the formula (1), it means the ionic conductivity ofan ion conductive fluoropolymer having a —SO₃M group in its molecule,obtained by hydrolytic treatment thereof.

Further, in the present invention, “a fluoromonomer A which gives apolymer having an alicyclic structure in its main chain” means a monomerwhich becomes a polymer having an alicyclic structure in its main chainby radical polymerization. Specifically, it includes two types, i.e. amonomer having an alicyclic structure in its molecule and a monomer forcyclopolymerization which has no alicyclic structure in its molecule butwhich forms an alicyclic structure as the polymerization reactionproceeds. Further, “having an alicyclic structure in its main chain”means that at least one of carbon atoms of the alicyclic structure inthe repeating unit is co-owned by the main chain.

The solid polymer electrolyte material of the present invention isconsidered to have high gas permeability, since it has a repeating unitbased on the above fluoromonomer A. Further, it has high ionicconductivity, since it has a repeating unit based on the fluoromonomer Bhaving a —SO₂X group. Further, fluorine atoms bonded to the carbon chainin such repeating units, contribute to the water repellency.Accordingly, if the solid polymer electrolyte material of the presentinvention is used as a constituting material for an electrode catalystlayer in a solid polymer fuel cell, the gas permeability can be improvedover a conventional material while maintaining high ionic conductivityand water repellency in the catalyst layer, whereby the cell output willbe improved, and yet, flooding will effectively be prevented, wherebysuch a high output can be obtained constantly.

The mechanism whereby the solid polymer fuel cell employing the solidpolymer electrolyte material of the present invention in an electrodecatalyst layer provides such a high output, is not clearly understood,but it is considered attributable to the alicyclic structure containedin the repeating unit based on the fluoromonomer A in the solid polymerelectrolyte material. Namely, it is considered that due to the alicyclicstructure, the solid polymer electrolyte material will be amorphous,whereby the gas permeability will be improved over the conventionalsolid polymer electrolyte material. Further, a gas diffusion electrodeprovided with a catalyst layer containing the solid polymer electrolyteof the present invention as a constituting material, or a polymerelectrolyte membrane formed from the solid polymer electrolyte materialof the present invention, is useful not only for a solid polymer fuelcell but also for an electrochemical process such as electrolysis ofsodium chloride.

Further, the present invention provides a liquid compositioncharacterized in that a solid polymer electrolyte material which is theabove-mentioned solid polymer electrolyte material and wherein the —SO₂Xgroup in the repeating unit based on the fluoromonomer B is a —SO₃Mgroup, is dissolved or dispersed in an organic solvent having a hydroxylgroup in its molecule. Here, M has the same meaning as M in the formula(1). The above liquid composition may contain water. When the boilingpoint of the above organic solvent is lower than the boiling point ofwater, by adding water to the liquid composition and distilling theorganic solvent off, it is possible to obtain a liquid compositionhaving the above-mentioned solid polymer electrolyte material dissolvedor dispersed in water, which contains substantially no organic solvent.

Among solid polymer electrolyte materials of the present invention, amaterial wherein the —SO₂X group is a —SO₃M group, can be dissolved orwell dispersed in the organic solvent having a hydroxyl group in itsmolecule. For example, if a liquid having fine particles of a catalystdispersed in a liquid composition obtainable by dissolving or dispersingin the above organic solvent a material having a —SO₃H group among solidpolymer electrolyte materials of the present invention, is used, acatalyst layer for a solid polymer fuel cell can easily be formed, and acatalyst layer excellent in gas permeability can be provided.

Further, the present invention provides a solid polymer fuel cellcomprising an anode, a cathode and a polymer electrolyte membranedisposed between the anode and the cathode, wherein the cathodecontains, as a constituting material, a solid polymer electrolytematerial wherein the fluoromonomer B has a —SO₃H group among theabove-mentioned electrolyte materials.

The reason why the solid polymer fuel cell using the solid polymerelectrolyte material of the present invention as a constituting materialfor the catalyst layer of the cathode, is capable of providing a highcell output constantly over a long period of time, is considered to besuch that the diffusibility of oxygen gas is improved while the ionicconductivity and water repellency in the catalyst layer of the cathodeare adequately secured, whereby the oxygen concentration overpotentialwill be reduced, and flooding will effectively be prevented.

Further, the solid polymer electrolyte material of the present inventionhas an alicyclic structure in its main chain and thus has a softeningtemperature higher than the conventional sulfonic acid polymer, and itis thus suitable for a high temperature operation of a fuel cell.

Further, the present invention provides a fluoropolymer which is acopolymer consisting essentially of a repeating unit of the followingformula (I) and a repeating unit based on a fluoromonomer D of thefollowing formula (II), wherein the content of the repeating unit basedon the fluoromonomer D is from 10 to 75 mol %, and the number averagemolecular weight is from 5,000 to 5,000,000:

 CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃M  (II)

Here, in the formulae (I) and (II), each of R^(f16) and R^(f17) whichmay be the same or different, is a fluorine atom or a trifluoromethylgroup, k′ is 0 or 1, Y is a fluorine atom or a trifluoromethyl group,and M has the same meaning as M in the formula (1).

Further, the present invention provides a fluoropolymer which is acopolymer consisting essentially of a repeating unit based onperfluoro(3-butenyl vinyl ether) and a repeating unit based on afluoromonomer D of the above formula (II), wherein the content of therepeating unit based on the fluoropolymer D is from 10 to 75 mol %, andthe number average molecular weight is from 5,000 to 5,000,000.

Further, the present invention provides a fluoropolymer which is acopolymer consisting essentially of a repeating unit based onperfluoro(2-methylene-4-methyl-1,3-dioxolane) and a repeating unit basedon a fluoropolymer D of the above formula (II), wherein the content ofthe repeating unit based on the fluoropolymer D is from 10 to 75 mol %,and the number average molecular weight is from 5,000 to 5,000,000.

These fluoropolymers of the present invention are those havingparticularly high gas permeability, among solid polymer electrolytematerials of the present invention, and they also have high ionicconductivity. Further, fluorine atoms bonded to carbon chains in theserepeating units, contribute to water repellency. Therefore, suchfluoropolymers of the present invention are useful as constitutingmaterials for the gas diffusion electrodes or polymer electrolytemembranes to be used for the above-mentioned electrochemical process.

In the fluoropolymer consisting essentially of the repeating unit of theformula (I) and the repeating unit based on the fluoromonomer D of theformula (II), if the content of the repeating unit based on thefluoromonomer D in the fluoropolymer is less than 10 mol %, the protonconductivity tends to be low, such being undesirable. On the other hand,if such a content exceeds 75 mol %, the gas diffusibility tends to below, such being undesirable. For the same reason, the content of therepeating unit based on the fluoromonomer D in the copolymer, is morepreferably from 15 to 60 mol %.

Further, if the number average molecular weight of this fluoropolymer isless than 5,000, the physical property such as the swelling degree tendsto change with time, whereby the durability tends to be inadequate. Onthe other hand, if the number average molecular weight exceeds5,000,000, preparation of a solution tends to be difficult. For the samereason, the number average molecular weight of the fluoropolymer is morepreferably from 10,000 to 3,000,000.

Further, as for the fluoropolymer consisting essentially of therepeating unit based on perfluoro(3-butenyl vinyl ether) and therepeating unit based on the fluoromonomer D of the formula (II), fromthe same viewpoint as the above-mentioned fluoropolymer containing therepeating unit of the formula (I), the content of the repeating unitbased on the fluoromonomer D in this fluoropolymer, is 10 to 75 vol %,and more preferably from 15 to 60 mol %. Further, the number averagemolecular weight of this fluoropolymer is 5,000 to 5,000,000, and alsomore preferably from 10,000 to 3,000,000.

Further, as for the fluoropolymer consisting essentially of therepeating unit based on perfluoro(2-methylene-4-methyl-1,3-dioxolane)and the repeating unit based on the fluoromonomer D of the formula (II),from the same viewpoint as the above-mentioned fluoropolymer containingthe repeating unit of the formula (I), the content of the repeating unitbased on the fluoromonomer D in this fluoropolymer, is 10 to 75 vol %,and more preferably from 15 to 60 mol %. Further, the number averagemolecular weight of this fluoropolymer is 5,000 to 5,000,000, and alsomore preferably from 10,000 to 3,000,000.

Now, the present invention will be described in detail with reference toan embodiment wherein the present invention is applied to a solidpolymer fuel cell.

The solid polymer fuel cell of the present invention has a constructioncomprising an anode, a cathode and a polymer electrolyte membranedisposed between the anode and the cathode. Each of the cathode and theanode which are gas diffusion electrodes, comprises a gas diffusionlayer and a catalyst layer adjacent to the gas diffusion layer. As theconstituting material of the gas diffusion layer, a porous materialhaving electron conductivity (such as a carbon cloth or carbon paper) isuseful.

The catalyst layer of the cathode mainly contains the above-mentionedsolid polymer electrolyte material (a —SO₃H type) of the presentinvention and a catalyst, in order to improve the cell output and inorder to improve the gas diffusibility and secure good ionicconductivity and water repellency in the catalyst layer, whereby thehigh cell output can be obtained constantly.

The solid polymer electrolyte material of the present invention to beincorporated to the catalyst layer of the cathode, is made of acopolymer comprising the repeating unit based on the fluoromonomer A andthe repeating unit based on the fluoromonomer B. both the fluoromonomerA and the fluoromonomer B are preferably perfluoromonomers. It isparticularly preferred that the fluoromonomer B is a compound of thefollowing formula (2), especially preferably a compound of the formula(6).CF₂═CFO(CF₂CFYO)_(k)(CF₂)_(m)SO₂X  (2)CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₂X  (6)

Here, in the formulae (2) and (6), k is an integer of from 0 to 2, m isan integer of from 1 to 12, k′ is 0 or 1, Y is a fluorine atom ortrifluoromethyl group, and X has the same meaning as X in the aboveformula (1). Thus, when both the fluoromonomer A and the fluoromonomer Bare perfluoromonomers, the water repellency and durability of theresulting solid polymer electrolyte material will be improved. Further,when the fluoromonomer B is a compound of the formula (2), the resultingsolid polymer electrolyte material will exhibits good ionicconductivity.

As mentioned above, the fluoromonomer A in the present inventionspecifically includes two types i.e. a monomer having an alicyclicstructure in its molecule and a monomer for cyclopolymerization. Therepeating unit based on the fluoromonomer A is preferably represented byany one of the following formulae (3) to (5):

Here, in the formula (3) each of p, q and r which are independent ofeach other, is 0 or 1, each of R^(f1) and R^(f2) which may be the sameor different, is a fluorine atom, a C₁₋₅ perfluoroalkyl group or a C₁₋₅perfluoroalkoxy group, and R^(f3) is a C₁₋₃ perfluoroalkylene group,which may have a C₁₋₅ perfluoroalkyl group or a C₁₋₅ perfluoroalkoxygroup, as a substituent.

Further, in the formula (4), s is 0 or 1, each of R^(f4), R^(f5), R^(f6)and R^(f7) which may be the same or different, is a fluorine atom or aC₁₋₅ perfluoroalkyl group, and R^(f8) is a fluorine atom, a C₁₋₅perfluoroalkyl group or a C₁₋₅ perfluoroalkoxy group, provided thatR^(f4) and R^(f5) may be connected to form a spiro ring when s=0.

Further, in the formula (5), each of R^(f9), R^(f10), R^(f11) andR^(f12) which may be the same or different, is a fluorine atom or a C₁₋₅perfluoroalkyl group.

The structure of the repeating unit of the above formula (3) can beformed from the monomer for cyclopolymerization, and theperfluoroalkylene group represented by R^(f3) may have a C₁₋₅perfluoroalkyl group or a C₁₋₅ perfluoroalkoxy group bonded as asubstituent. Further, when subjected to cyclopolymerization, in theformula (3), when q=o, r=1 and when q=1, r=0, Specifically, such arepeating unit includes, for example, those represented by the followingformulae (7) to (22):

Further, the structure of the repeating unit of the above formula (4)can be formed from a monomer having an alicyclic structure in itsmolecule. Specifically, such a repeating unit includes, for example,those represented by the following formulae (23) to (32). Further, in acase where in the structure of the repeating unit of the formula (4),when the spiro ring formed by R^(f4) and R^(f5) when s=0, is a 4- to6-membered ring, such a ring may contain an ether oxygen atom as anelement constituting the ring, and such a ring may have a perfluoroalkylgroup bonded as a substituent. Such a structure of the repeating unitbased on the monomer having an alicyclic structure in its molecule, may,for example, be one represented by the following formula (33).

Further, the structure of the repeating unit of the above formula (5)can also be formed from a monomer having an alicyclic structure in itsmolecule. Specifically, such a repeating unit includes, for example,those represented by the following formulae (34) to (36).

Among repeating units based on the fluoromonomer A, preferred is atleast one member selected from the group consisting of repeating unitsof the formulae (7), (23), (24), (29) and (34) Monomers (fluormonomersA) to be used as starting materials to introduce such repeating unitsinto copolymers constituting solid polymer electrolyte materials, are,respectively, as follows. Formula (7): perfluoro(3-butenyl vinyl ether),formula (23): perfluoro(2,2-dimethyl-1,3-dioxole), formula (24):perfluoro(1,3-dioxole), formula (29):2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole, and formula (34):perfluoro(2-methylene-4-methyl-1,3-dioxolane).

A solid polymer electrolyte material made of a copolymer comprising sucha repeating unit and a repeating unit based on the monomer of the aboveformula (6), is particularly preferred, since it has high ionicconductivity and is excellent in water repellency and oxygenpermeability. Particularly when one having a —SO₃H group among the abovecopolymers, is incorporated to the catalyst layer of a cathode in asolid polymer fuel cell, the output of the resulting solid polymer fuelcell can be made higher than ever.

Among solid polymer electrolyte materials of the present invention,preferred is a fluoropolymer which is a copolymer consisting essentiallyof the repeating unit of the above formula (I) and the repeating unitbased on the fluoromonomer D of the formula (II), wherein the content ofthe repeating unit based on the fluoromonomer D is from 10 to 75 mol %,and the number average molecular weight is from 5,000 to 5,000,000.Further, also preferred is a fluoropolymer which is a copolymerconsisting essentially of the repeating unit based onperfluoro(3-butenyl vinyl ether) and the repeating unit based on thefluoromonomer D of the above formula (II), wherein the content of therepeating unit based on the fluoromonomer D is from 10 to 75 mol %, andthe number average molecular weight is from 5,000 to 5,000,000. Stillfurther, also preferred is a fluoropolymer which is a copolymerconsisting essentially of the repeating unit based onperfluoro(2-methylene-4-methyl-1,3-dioxolane) and the repeating unitbased on the fluoromonomer D of the above formula (II), wherein thecontent of the repeating unit based on the fluoromonomer D is from 10 to75 mol %, and the number average molecular weight is from 5,000 to5,000,000.

And, further, when the solid polymer electrolyte material of the presentinvention is used as an electrolyte material for a catalyst layer of acathode in a solid polymer fuel cell as in this embodiment, if the —SO₂Xgroups in the copolymer comprising the repeating unit based on thefluoromonomer A and the repeating unit based on the fluoromonomer B, areother than —SO₃H groups, such a material is preliminarily subjected toacid form-conversion treatment to convert them to —SO₃H groups and thenused. The hydrolytic treatment of the —SO₂F groups in the precursor maybe carried out by using, for example, an aqueous solution of a base suchas NaOH or KOH or a mixed solution of such a base in water and awater-soluble organic solvent to convert them to —SO₃Na groups or —SO₃Kgroups. Further, the acid form-conversion treatment may be carried outby using, for example, an aqueous solution of e.g. hydrochloric acid,nitric acid or sulfuric acid, to convert the —SO₃Na or —SO₃K groups to—SO₃H groups.

Further, the softening temperature of the copolymer as the solid polymerelectrolyte material of the present invention is preferably at least100° C. Here, the softening temperature of the solid polymer electrolytematerial in the present invention, means a temperature at which theelastic modulus of the solid polymer electrolyte material starts toabruptly decrease when in an evaluation test of the dynamicviscoelasticity of the solid polymer electrolyte material, the elasticmodulus is measured while gradually raising the temperature of the solidpolymer electrolyte material from in the vicinity of room temperature.Accordingly, the softening temperature in the present invention isdifferent from the glass transition temperature usually obtained fromthe value of tan δ and represents a temperature which is usuallyobserved in a temperature region lower than the glass transitiontemperature.

Specifically, this softening temperature can be measured by apenetration method by means of a quartz probe having a diameter of 1 mmby using a thermal mechanical analyzer (TMA). Namely, the solid polymerelectrolyte material to be measured is cast from its solution to form afilm, and the quartz probe is contacted to this film in a directionnormal to the film surface, and the temperature is raised at atemperature raising rate of from 1 to 10° C./min, whereby thetemperature at which the thickness of the film starts to abruptlydecrease, is measured as the softening temperature, as observed by thepenetration of the probe into the film. It has been preliminarilyconfirmed that the value of the softening temperature obtained by thismethod agrees to the value of the temperature at which the abruptdecrease in the elastic modulus starts to be observed in theabove-described profile of the temperature dependency of the elasticmodulus of the polymer. Further, in a case where the load of the probeexerted to the film is too small, the thermal expansion of the film willbe observed, but by optimizing the load, the degree of penetration ofthe probe at the softening temperature of the film, can be measuredwithout any problem.

The operation temperature of a solid polymer fuel cell is usually atmost 80° C. Therefore, if the softening temperature of the solid polymerelectrolyte material contained in the catalyst layer is at least 100°C., a change with time in the physical property such as the swellingdegree of the solid polymer electrolyte material in the catalyst layerduring the operation of the cell, can be suppressed. Therefore, thedurability of the solid polymer electrolyte material in the catalystlayer during the operation of the cell, will be improved. Further, ifthe solid polymer electrolyte material having a softening point of atleast 100° C., is used as a material for the catalyst layer of the anodeand for the polymer electrolyte membrane, in addition to the catalystlayer of the cathode, the durability of the electrolyte material in thecatalyst layer of the anode or of the polymer electrolyte membrane,during the operation of the cell will be improved in the same manner asdescribed above, and accordingly, the cell life can be improved.

Further, in such a case, by using the solid polymer electrolyte materialhaving a softening point of at least 100° C. also for the polymerelectrolyte membrane, the operational temperature of a conventionalsolid polymer fuel cell can be made higher than 80° C. It is therebypossible to effectively utilize the exhaust heat of the cell, and at thesame time, the temperature control of the cell during the operation willbe easier, since heat removal of the cell becomes easy. Further, in thiscase, it becomes possible to reduce the catalyst poisoning due to e.g.carbon monoxide contained in the anode reaction gas, and it becomespossible to improve the cell life also from this viewpoint. Further,also in a case where the solid polymer electrolyte material of thepresent invention is used as a solid acid catalyst, the softeningtemperature can be made high, whereby the reaction temperature can bemade high, and the desired reaction can be proceeded in a highertemperature region.

In order to have a high softening temperature and have practicalstrength as a membrane, particularly preferred is a fluoropolymer whichis a copolymer consisting essentially of a repeating unit of the formula(I), a repeating unit based on the fluoromonomer D of the formula (II)and a repeating unit based on tetrafluoroethylene, wherein the repeatingunit of the formula (I) is from 10 to 70 mol %, preferably from 20 to 60mol % the repeating unit based on tetrafluoroethylene is from 10 to 70mol %, preferably from 20 to 60 mol %, the content of the repeating unitbased on the fluoromonomer D of the formula (II) is from 10 to 40 mol %,preferably from 10 to 30 mol %, and the number average molecular weightis from 5,000 to 5,000,000.

Further, the solid polymer electrolyte material of the present inventionpreferably has A_(R) of from 0.5 to 2.5 meq/g dry resin (hereinaftersimply represented by meq/g). If A_(R) of the solid polymer electrolytematerial is less than 0.5 meq/g, the water content of the solid polymerelectrolyte material tends to decrease, and its ionic conductivity tendsto be low, and if such a solid polymer electrolyte material is used as aconstituting material for the catalyst layer of an electrode in a solidpolymer fuel cell, it tends to be difficult to obtain an adequate celloutput. On the other hand, if A_(R) of the solid polymer electrolytematerial exceeds 2.5 meq/g, the density of ion exchange groups in thesolid polymer electrolyte material increases, whereby the strength ofthe solid polymer electrolyte material tends to be low. Further, if sucha material is used as a constituting material for a catalyst layer of anelectrode in a solid polymer fuel cell, the water content tends to betoo high, whereby the gas diffusion or water drainage in the catalystlayer tends to be low, and flooding is likely to occur. For the samereason, A_(R) of the solid polymer electrolyte material of the presentinvention is more preferably from 0.7 to 2.0 meq/g, still furtherpreferably from 0.9 to 1.5 meq/g.

Further, the number average molecular weight of the solid polymerelectrolyte material of the present invention is not particularlylimited, and the degree of polymerization of the copolymer may bechanged depending upon the particular purpose to suitably set themolecular weight. However, in a case where it is used as a constitutingmaterial for a catalyst layer of a cathode in a solid polymer fuel cell,as in the present embodiment, the number average molecular weight ispreferably from 5,000 to 5,000,000, more preferably from 10,000 to3,000,000. If the number average molecular weight of the solid polymerelectrolyte material is less than 5,000, the physical property such asthe swelling degree tends to change with time, whereby the durabilitytends to be inadequate. On the other hand, if the number averagemolecular weight exceeds 5,000,000, preparation of a solution tends tobe difficult.

The ratio (mass ratio) of the repeating unit based on the fluoromonomerA to the repeating unit based on the fluoromonomer B in the solidpolymer electrolyte material of the present invention is notparticularly limited, and it may be suitably set depending upon theparticular purpose. However, when the material is used as a constitutingmaterial for a catalyst layer of a cathode in a solid polymer fuel cell,as in the present embodiment, the ratio is preferably selected to meetthe above range of A_(R).

Further, to the solid polymer electrolyte material of the presentinvention, in addition to the repeating unit based on the fluoromonomerA and the repeating unit based on the fluoromonomer B, other repeatingunits may be incorporated as repeating units constituting the solidpolymer electrolyte material, as the case requires, such as foradjustment of the mechanical strength. Monomers giving such otherrepeating units are not particularly limited, and, for example,tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride,hexafluoropropylene, trifluoroethylene, vinyl fluoride, ethylene andfluorovinyl compounds of the following formulae (37) to (40) may bementioned. Further, for the purpose of improving the mechanical strengthof the resulting copolymer, among these monomers, it is preferred toemploy tetrafluoroethylene from the viewpoint of the high activity tothe polymerization reaction, durability (perfluoro structure) andavailability.CH₂═CHR^(f13)  (37)CH₂═CHCH₂R^(f13)  (38)CF₂═CFO(CF₂CFWO)_(a)R^(f15)  (39)CF₂═CFOR^(f14)Z  (40)

Here, in the formulae (37) and (38), R^(f13) represents a C₁₋₁₂perfluoroalkyl group. Further, in the formula (39), a is an integer offrom 0 to 3, W is a fluorine atom or a trifluoromethyl group, andR^(f15) is a C₁₋₁₂ perfluoroalkyl group having a straight chain orbranched structure. Further, in the formula (40), R^(f14) is a C₁₋₁₂perfluoroalkylene group having a straight chain or branched structure,which may contain ether oxygen atoms, and Z is —CN, —COOR^(f16) (whereinR^(f16) is a C₁₋₆ alkyl group) or —COF.

Further, in a case where in addition to the repeating unit based on thefluoromonomer A and the repeating unit based on the fluoromonomer B,other repeating units are incorporated to the solid polymer electrolytematerial of the present invention, the content of such other repeatingunits may suitably be determined depending upon the particular purposeof the solid polymer electrolyte material. In a case where the materialis used as a constituting material for a catalyst layer of a cathode ina solid polymer fuel cell, as in the present embodiment, the content ofsuch other repeating units in the copolymer constituting the solidpolymer electrolyte material is preferably less than 35 mass %. If thisvalue exceeds 35 mass %, the effect for increasing the output of thefuel cell tends to be small.

Further, among fluorovinyl ether compounds represented by the formula(39), it is preferred to employ fluorovinyl ether compounds of thefollowing formulae (41) to (43) In the following formulae (41) to (43),b is an integer of from 1 to 8, d is an integer of from 1 to 8, and e is2 or 3.CF₂═CFO(CF₂)_(b)CF₃  (41)CF₂═CFOCF₂CF(CF₃)O(CF₂)_(d)CF₃  (42)CF₂═CF(OCF₂CF(CF₃))_(e)O(CF₂)₂CF₃  (43)

A catalyst incorporated to the catalyst layer of a cathode in thepresent invention, is not particularly limited. For example, a catalysthaving a platinum group metal such as platinum or its alloy supported oncarbon, is preferred.

Further, in the catalyst layer of a cathode, the range of the mass ratioof the catalyst to the solid polymer electrolyte material is preferablysuch that mass of the catalyst (total mass of metal and the carboncarrier): mass of the solid polymer electrolyte material=20:80 to 95:5,more preferably 30:70 to 90:10.

Here, if the content of the catalyst to the solid polymer electrolytematerial is too low, the amount of the catalyst is small, whereby thereaction sites tend to be deficient. Further, the covering layer of thesolid polymer electrolyte material which covers the catalyst, tends tobe thick, whereby the diffusion rate of the reaction gas in the solidpolymer electrolyte material tends to be small. Further, pores requiredfor the diffusion of the reaction gas are likely to be clogged with theresin, whereby a phenomenon of flooding is likely to occur. On the otherhand, if the content of the catalyst to the solid polymer electrolytematerial is too high, the amount of the solid polymer electrolytematerial covering the catalyst tends to be inadequate to the catalyst,whereby the reaction sites tend to be less, and the cell output tends tobe low. Further, the solid polymer electrolyte material functions alsoas a binder for the catalyst layer and an adhesive between the catalystlayer and the polymer electrolyte membrane, but such a function tends tobe inadequate, whereby the structure of the catalyst layer tends to behardly maintained stably.

The construction of the catalyst layer of an anode in this fuel cell isnot particularly limited, and it can be constituted in the same manneras a catalyst layer of an anode in a conventional solid polymer fuelcell. It may contain the solid polymer electrolyte material of thepresent invention, and it may contain other resin.

The thickness of the catalyst layer of the cathode and anode in thepresent invention is preferably from 1 to 500 μm, more preferably from 5to 100 μm. Further, the catalyst layer in the present invention maycontain a water repellent agent such as PTFE, as the case requires.However, the water repellent agent is an insulating material, andaccordingly, its amount is preferably as small as possible, and it isusually preferably at most 30 mass %.

The polymer electrolyte membrane to be used for the solid polymer fuelcell of the present invention is not limited so long as it is an ionexchange membrane showing good proton conductivity in a wet state, but aperfluorinated membrane is preferred from the viewpoint of durability.As the solid polymer material constituting the polymer electrolytemembrane, the solid polymer electrolyte material of the presentinvention may, for example, be employed, or an ion exchange resin whichis used in a conventional solid polymer fuel cell, may be employed.

Now, an example of the method for producing a fluoropolymer to be usedfor a solid polymer electrolyte material in the present invention, willbe described. Firstly, as the fluoromonomer B, one having a SO₂F groupor a —SO₂Cl group, is used. The polymerization reaction of thefluoromonomer A and the fluoromonomer B is not particularly limited solong as it is carried out under a condition where radicals will beformed. For example, it may be carried out by bulk polymerization,solution polymerization, suspension polymerization, emulsionpolymerization or polymerization in liquid or super critical carbondioxide. The method for generating radicals is not particularly limited,and for example, a method of irradiating a radiation such as ultravioletrays, γ-rays or electron rays, may be employed, or a method of using aradical initiator to be used in usual radical polymerization, may beemployed. The reaction temperature for the polymerization reaction isalso not particularly limited, and for example, it is usually from 15 to150° C. In a case where a radical initiator is used, the radicalinitiator may, for example, be a bis(fluoroacyl)peroxide, abis(chlorofluoroacyl)peroxide, a dialkylperoxy dicarbonate, a diacylperoxide, a peroxy ester, an azo compound or a persulfate.

In a case where solution polymerization is carried out, the solvent tobe used usually preferably has a boiling point of from 20 to 350° C.from the viewpoint of handling efficiency, more preferably has a boilingpoint of from 40 to 150° C. And, in the solvent, predetermined amountsof the fluoromonomer and the fluorovinyl compound are put, and a radicalinitiator is added to let radicals form to carry out the polymerization.Here, a useful solvent may, for example, be (i) apolyfluorotrialkylamine compound such as perfluorotributylamine orperfluorotripropylamine, (ii) a fluoroalkane such as perfluorohexane,perfluorooctane, perfluorodecane, perfluorododecane,perfluoro(2,7-dimethyloctane), 2H,3H-perfluoropentane,1H-perfluorohexane, 1H-perfluorooctane, 1H-perfluorodecane,1H,4H-perfluorobutane, 1H,1H,1H,2H,2H-perfluorohexane,1H,1H,1H,2H,2H-perfluorooctane, 1H,1H,1H,2H,2H-perfluorodecane,3H,4H-perfluoro(2-methylpentane) or 2H,3H-perfluoro(2-methylpentane),(iii) a chlorofluoroalkane such as3,3-dichloro-1,1,1,2,2-pentafluoropropane,1,3-dichloro-1,1,2,2,3-pentafluoropropane or1,1-dichloro-1-fluoroethane, (iv) a fluoroolefin having no double bondat the terminal of the molecular chain, such as a dimer ofhexafluoropropene or a trimer of hexafluoropropene, (v) apolyfluorocycloalkane such as perfluorodecalin, perfluorocyclohexane,perfluoro(1,2-dimethylcyclohexane), perfluoro(1,3-dimethylcyclohexane),perfluoro(1,3,5-trimethylcyclohexane) or perfluorodimethylcyclobutane(irrespective of the structural isomerism), (vi) a polyfluorocyclicether compound such as perfluoro(2-butyltetrahydrofuran), (vii) ahydrofluoroether such as n-C₃F₇OCH₃, n-C₃F₇OCH₂CF₃, n-C₃F₇OCHFCF₃,n-C₃F₇OC₂H₆, n-C₄F₉OCH₃, iso-C₄F₉OCH₃, n-C₄F₉OC₂H₅, iso-C₄F₉OC₂H₅,n-C₄F₉OCH₂CF₃, n-C₅F₁₁OCH₃, n-C₆F₁₃OCH₃, n-C₅F₁₁OC₂H₅,CF₃OCF(CF₃)CF₂OCH₃, CF₃OCHFCH₂OCH₃, CF₃OCHFCH₂OC₂H₅ orn-C₃F₇OCF₂CF(CF₃)OCHFCF₃, (viii) a fluorine-containing low molecularweight polyether, or (ix) tert-butanol or the like. These solvents maybe used alone or in combination as a mixture of two or more of them.

Another example of the solvent to be used for the solutionpolymerization may be a chlorofluorocarbon such as1,1,2-trichloro-1,2,2-trifluoroethane,1,1,1-trichloro-2,2,2-trifluoroethane,1,1,1,3-tetrachloro-2,2,3,3-tetrafluoropropane or1,1,3,4-tetrachloro-1,2,2,3,4,4-hexafluorobutane. Such achlorofluorocarbon may be technically useful, but its use is notdesirable when the influence to the global environment is taken intoconsideration.

The copolymer obtainable by the polymerization has a —SO₂F group or a—SO₂Cl group, and accordingly, it is subjected to hydrolysis or thatfollowed by acid-modification treatment, as the case requires to convertthe group to a —SO₃M group.

Now, with respect to the solid polymer fuel cell of the presentinvention, an example of the method for its production will bedescribed, and at the same time, a preferred embodiment in which theliquid composition of the present invention is applied to the solidpolymer fuel cell, will be described. The method for preparing a gasdiffusion electrode having a catalyst layer of a cathode and anode forthe solid polymer fuel cell of the present invention, is notparticularly limited, and it can be prepared by a conventional method.

For example, the catalyst layer of a cathode can be formed by using acoating fluid for forming a catalyst layer, which is prepared by mixinga catalyst with a liquid composition having the solid polymerelectrolyte material of the present invention having a —SO₃H group,dissolved or dispersed in a solvent having a hydroxyl group in itsmolecule.

The solid polymer electrolyte material of the present invention can bewell dissolved or dispersed in an organic solvent having a hydroxylgroup, in a case where it has a —SO₃M group. The organic solvent havinga hydroxyl group is not particularly limited, but it is preferably anorganic solvent having an alcoholic hydroxyl group. The organic solventhaving an alcoholic hydroxyl group may, for example, be methanol,ethanol, 1-propanol, 2-propanol, 2,2,2-trifluoroethanol,2,2,3,3,3-pentafluoro-1-propanol, 2,2,3,3-tetrafluoro-1-propanol,4,4,5,5,5-pentafluoro-1-pentanol, 1,1,1,3,3,3-hexafluoro-2-propanol,3,3,3-trifluoro-1-propanol, 3,3,4,4,5,5,6,6,6-nonafluoro-1-hexanol or3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol. Further, as anorganic solvent other than an alcohol, an organic solvent having acarboxyl group such as acetic acid, may also be used.

Here, as the organic solvent having a hydroxyl group, theabove-mentioned solvents may be used alone or in combination as amixture of two or more of them. Further, it may be used as mixed withwater or with other fluorine-containing solvents. As such otherfluorine-containing solvents, fluorine-containing solvents exemplifiedas preferred fluorine-containing solvents in the solution polymerizationreaction in the production of the above-described solid polymerelectrolyte material, may be mentioned as examples. When the organicsolvent having a hydroxyl group is used as a mixed solvent with water oranother fluorine-containing solvent, the content of the organic solventhaving a hydroxyl group is preferably at least 10%, more preferably atleast 20%, based on the total mass of the solvent. In such a case, thesolid polymer electrolyte material may be dissolved or dispersed in themixed solvent from the beginning. Otherwise, firstly, the solid polymerelectrolyte material may be dissolved or dispersed in the organicsolvent having a hydroxyl group, and then, water or anotherfluorine-containing solvent may be mixed thereto. Further, dissolutionor dispersion of the solid polymer electrolyte material in such asolvent is preferably carried out within a temperature range of from 0to 250° C., more preferably within a range of from 20 to 150° C. underatmospheric pressure or under such a condition as closed and pressurizedby e.g. an autoclave.

The content of the solid polymer electrolyte material in the liquidcomposition of the present invention is preferably from 1 to 50%, morepreferably from 3 to 30%, based on the total mass of the liquidcomposition. If the content of the solid polymer electrolyte material isless than 1%, when a catalyst is mixed to this liquid to prepare acoating solution, which is used for preparation of a catalyst layer of acathode, the number of coating steps will have to be increased toprepare a catalyst layer having a desired thickness, or a large amountof an organic solvent is contained in such a coating solution, suchbeing costly, and it takes time to remove such an organic solvent,whereby the production operation can hardly be efficiently carried out.On the other hand, if the content of the solid polymer electrolytematerial exceeds 50%, the viscosity of the liquid composition tends tobe too high, whereby handling tends to be difficult.

Further, to the liquid composition, in addition to the solid polymerelectrolyte material of the present invention, a resin which is anothersolid polymer electrolyte material, may be incorporated. In such a case,with a view to sufficiently securing water repellency and gasdiffusibility in the catalyst layer obtained by using the liquidcomposition as the starting material, the content of the solid polymerelectrolyte material of the present invention in the liquid compositionis preferably at least 20%, more preferably at least 50%, based on thetotal mass of all the solid polymer electrolyte materials in the liquidcomposition.

The catalyst layer of a cathode can be prepared by coating a coatingliquid for forming a catalyst layer, prepared by mixing a catalystcomposed of fine particles of e.g. carbon black having platinumsupported thereon, to the liquid composition of the present invention,on a polymer electrolyte membrane, a gas diffusion layer or a supportplate so that the thickness will be uniform, the solvent is removed bydrying, followed by hot pressing as the case requires. A coating liquidfor a catalyst layer may also be prepared as follows. A liquidcomposition of the present invention is mixed with a catalyst of fineparticles, the mixture thus obtained is dried and the dried solid isdispersed in another solvent which is usually selected from theabove-mentioned alcoholic solvents, sometimes mixed with water. In sucha manner, a catalyst layer of a cathode excellent in water repellencyand gas diffusibility can be obtained. Especially when a coatingsolution is prepared from a liquid composition containing the solidpolymer electrolyte material having the softening temperature of thepolymer itself being at least 100° C., and then a catalyst layer isprepared therefrom, the gas diffusibility in the layer will beremarkably improved. It is considered that if the softening temperatureof the solid polymer electrolyte material is at least 100° C., when thesolvent is gradually evaporated from the coating solution, the solidpolymer electrolyte material scarcely undergoes shrinkage, whereby poreshaving a proper size will be formed in the interior of the solid polymerelectrolyte material or among agglomerates of catalyst particles coatedby the solid polymer electrolyte material. Further, the catalyst layerof an anode can be formed in the same manner as the above catalyst layerof a cathode. The coating solution for forming the catalyst layer of ananode may be prepared by using the liquid composition of the presentinvention or by using a liquid having a conventional solid polymerelectrolyte material dissolved or dispersed in a prescribed solvent.

By interposing the prepared catalyst layer of a cathode and the catalystlayer of an anode between a polymer electrolyte membrane and a gasdiffusion layer, a solid polymer fuel cell can be prepared. Here, whenthe catalyst layer is formed on the polymer electrolyte membrane, aseparately prepared gas diffusion layer may, for example, be placed orbonded on the catalyst layer. Otherwise, when the catalyst layer isformed on a gas diffusion layer to preliminarily form a gas diffusionelectrode, a separately prepared polymer electrolyte membrane may bedisposed or bonded on the catalyst layer. Further, when a catalyst layeris formed on a support plate, it may be transferred to a separatelyprepared polymer electrolyte membrane, then the support plate is peeled,and a separately prepared gas diffusion layer is disposed or bonded onthe catalyst layer.

Bonding between the polymer electrolyte membrane and the catalyst layer,or the catalyst layer and the gas diffusion layer, may be carried out,for example, by hot press or roll press. At that time, the two may bebonded without heating by means of an adhesive such as aperfluorosulfonic acid polymer solution or the like, as the adhesive.

Further, as mentioned above, the polymer electrolyte membraneconstituting a cell, may be prepared by using the solid polymerelectrolyte material of the present invention.

In the foregoing, a preferred embodiment of the present invention hasbeen described in detail, but the present invention is by no meansrestricted to the above-described embodiment. For example, in the aboveembodiment, a solid polymer fuel cell has been described in a case wherea gas containing hydrogen as the main component is used as the anodereaction gas. However, the solid polymer fuel cell of the presentinvention may, for example, be one having a construction such that asthe anode reaction gas, methanol gas is directly introduced to theanode.

Further, in the above embodiment, the liquid composition containing thesolid polymer electrolyte material of the present invention is used fora catalyst layer of an electrode in a solid polymer fuel cell. However,it can be used for other applications. For example, when a membrane isformed by using the solid polymer electrolyte material of the presentinvention, it can be used in various electrochemical processes 1) as acation permselective membrane to be used for e.g. electrolysis of sodiumchloride, 2) as a membrane for electrolysis of water, 3) as a protonpermselective membrane to be used, for production of hydrogen peroxide,for production of ozone or for recovery of waste acid, or 4) as a cationexchange membrane for electrodialysis to be used for desalination orsalt production.

Further, for other than the electrochemical processes, a membrane may beformed by using the solid polymer electrolyte material of the presentinvention, and it may, for example, be used as a membrane for diffusiondialysis to be used for separation and purification of an acid, a baseand a salt, as a charged porous membrane (a charged reverse osmosismembrane, a charged ultrafiltration membrane, a charged microfiltrationmembrane, etc.) for the separation of a protein, as a dehumidifyingmembrane or as a humidifying membrane. Further, the solid polymerelectrolyte material of the present invention can be used also as e.g. apolymer electrolyte for a lithium ion cell, a solid acid catalyst, acation exchange resin, a sensor employing a modified electrode, an ionexchange filter for removing a trace amount of ions in air, or anactuator.

Now, the solid polymer electrolyte material, the liquid composition, thesolid polymer fuel cell and the fluoropolymer, of the present invention,will be described in further detail with reference to Examples andComparative Examples. However, it should be understood that the presentinvention is by no means restricted to such Examples. In the followingExamples and Comparative Examples, the following compounds will berepresented by the following abbreviations.

-   -   PSVE: CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F,    -   PSVE-H: CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₃H,    -   BVE: Perfluoro(3-butenyl vinyl ether),    -   MMD: Perfluoro(2-methylene-4-methyl-1,3-dioxolane),    -   PDD: Perfluoro(2,2-dimethyl-1,3-dioxole),    -   TFE: Tetrafluoroethylene    -   IPP: (CH₃)₂CHOC(═O)OOC(═O)OCH(CH₃)₂,    -   HCFCl41b: CH₃CCl₂F,    -   HCFC225cb: CClF₂CF₂CHClF.

PREPARATION EXAMPLE 1 PDD/PSVE-H Copolymer 1

Into a stainless steel autoclave having a capacity of 0.2 l, 26.0 g ofPDD, 127.89 of PSVE and 0.46 g of IPP were put, and the gas in theautoclave was purged by nitrogen, and thereafter, nitrogen wasintroduced so that the total pressure would be 0.3 MPa (gauge pressure).Then, the temperature in the autoclave was raised to be 40° C., andpolymerization was initiated while stirring the content. After 10 hoursfrom the initiation of the polymerization, the interior of the autoclavewas cooled, and the gas in the interior was purged to stop thepolymerization. After diluting with HCFC225cb, hexane was the resultingmixture was poured into hexane to precipitate the polymer, which waswashed twice with hexane and further once with HCFCl₄₁b. Afterfiltration, vacuum drying was carried out at 80° C. for 16 hours toobtain 41.6 g of a white polymer. The content of sulfur was obtained byan elemental analysis, and the molar ratio (PDD/PSVE) of the repeatingunit based on PDD to the repeating unit based on PSVE in the polymer andA_(R) were obtained, whereby PDD/PSVE=56.5/43.5, and A_(R)=1.31 meq/g.Further, the average molecular weight of the polymer was measured byGPC, whereby the number average molecular weight as calculated aspolymethyl methacrylate was 33,000. The weight average molecular weightwas 56,000.

Then, the obtained polymer was hydrolyzed in a KOH solution dissolved ina water/methanol mixture and then immersed in a dilute sulfuric acidaqueous solution for acid-form conversion treatment. Then, the polymerwas washed with deionized water and dried, and then dissolved in ethanolto obtain a transparent ethanol solution containing 10 mass % of thepolymer (PDD/PSVE-H copolymer 1).

A cast film was prepared by using the ethanol solution of the abovepolymer, and the softening point of the polymer was measured by theabove-mentioned penetration method by means of a quartz probe having adiameter of 1 mm. Firstly, a mixed solution comprising 10 parts by massof the ethanol solution of the polymer and 2 parts by mass of butanol,was prepared, and this solution was used for cast-film forming at roomtemperature and dried at 160° C. for 30 minutes to obtain a cast filmhaving a thickness of about 200 μm. Then, the obtained cast film was setin TMA (manufactured by Mack Science Company). And, while raising thetemperature of is the cast film at a temperature raising rate of 5°C./min, a vibration load based on a sin curve of 0.2 Hz (load vibrationrange: 1 to 6 g, average load: 3.59) was exerted to the contact portionbetween the cast film and the quartz probe having a diameter of 1 mm,whereby the change in the thickness of the cast film was measured. And,the temperature at which the thickness of the film started to abruptlydecrease due to penetration of the probe into the cast film, wasmeasured as the softening point. As a result, the softening point ofthis polymer was 150° C.

PREPARATION EXAMPLE 2 PDD/PSVE-H Copolymer 2

Into a stainless steel autoclave having a capacity of 0.2 l, 36.49 ofPDD, 123.1 g of PSVE and 0.489 of IPP were put, and polymerization wasinitiated in the same manner as in Preparation Example 1. After 3.2hours from the initiation of the polymerization, the interior of theautoclave was cooled, and the gas in the interior was purged to stop thepolymerization. After diluting with HCFC225cb, the diluted product wasput into hexane for precipitation, and the precipitate was washed twicewith hexane and further once with HCFCl₄₁b. After filtration, vacuumdrying was carried out at 80° C. for 16 hours to obtain 25.39 of a whitepolymer. With respect to the obtained polymer, hydrolysis and acid-formconversion treatment were carried out in the same manner as inPreparation Example 1 to obtain a PDD/PSVE-H copolymer 2, and the samecharacterization as in Preparation 1 was carried out. As a result,PDD/PSVE=69.8/30.2, A_(R)=0.99 meq/9, the number average molecularweight as calculated as polymethyl methacrylate: 58,000, the weightaverage molecular weight: 95,000, and the softening point: 180° C.

PREPARATION EXAMPLE 3 BVE/PSVE-H Copolymer 1

In a nitrogen atmosphere, 120.09 of BVE, 128.59 of PSVE and 0.76 g ofIPP were put into a flask having a capacity of 300 ml, and thetemperature in the flask was raised to be 40° C. to initiatepolymerization while stirring the content. After 16.7 hours from theinitiation of the polymerization, the interior of the flask was cooledto stop the polymerization, and the product was put into hexane toprecipitate the polymer, which was further washed three times withhexane. After filtration, vacuum drying was carried out at 80° C. for 16hours to obtain 47.89 of a white polymer. With respect to the obtainedpolymer, hydrolysis and acid-form conversion treatment were carried outin the same manner as in Preparation Example 1 to obtain a BVE/PSVE-Hcopolymer 1, and the same characterization as in Preparation Example 1was carried out. As a result, BVE/PSVE=67.0/33.0, A_(R)=0.99 meq/g, thenumber average molecular weight as calculated as polymethylmethacrylate: 29,000, the weight average molecular weight: 42,000, andthe softening temperature: 110° C.

PREPARATION EXAMPLE 4 BVE/PSVE-H Copolymer 2

In a nitrogen atmosphere, 150.0 g of BVE, 103.0 g of PSVE and 0.77 g ofIPP were put into a flask having a capacity of 300 ml, andpolymerization was initiated in the same manner as in PreparationExample 3. After 10.7 hours from the initiation of the polymerization,the interior of the flask was cooled to stop the polymerization, and theproduct was put into hexane to precipitate the polymer, which was washedthree times with hexane and further once with HCFCl₄₁b. Afterfiltration, vacuum drying was carried out at 80° C. for 16 hours toobtain 38.0 g of a white polymer. With respect to the obtained polymer,hydrolysis and acid-form conversion treatment were carried out in thesame manner as in Preparation Example 1 to obtain a BVE/PSVE-H copolymer2, and the same characterization as in Preparation Example 1 was carriedout. As a result, BVE/PSVE=76.1/23.9, A_(R)=0.75 meq/9, the numberaverage molecular weight as calculated as polymethyl methacrylate:38,000, the weight average molecular weight: 53,000, and the softeningtemperature: 110° C.

PREPARATION EXAMPLE 5 TFE/PSVE-H Copolymer

A TFE/PSVE copolymer which has heretofore been employed as a materialfor a catalyst layer of an electrode in a solid polymer fuel cell or asa material for a polymer electrolyte membrane, was prepared by a knownmethod. With respect to the obtained polymer, hydrolysis and acid-formconversion treatment were carried out in the same manner as inPreparation Example 1 to obtain a TFE/PSVE-H copolymer, and the samecharacterization as in Preparation Example 1 was carried out, wherebyTFE/PSVE=82.2/17.8, A_(R)=1.1 meq/g, and the softening temperature: 80°C.

PREPARATION EXAMPLE 6 MMD/PSVE-H Copolymer 1

Into a 0.2 l autoclave, 0.68 g of IPP, 207.1 g of PSVE and 20.0 g of MMDwere put, and after degassing under reduced pressure, raising pressurewith nitrogen followed by purging was carried out three times, whereuponnitrogen was introduced so that the total pressure would be 0.12 MPa(gauge pressure). Then, the temperature of the autoclave was raised to40° C., and the reaction was carried out for 2.5 hours. Thepolymerization solution was put into hexane for precipitation, followedby further washing three times with hexane. Vacuum drying was carriedout at room temperature overnight, and further vacuum drying was carriedout at 80° C. overnight to obtain 16.9 g of a polymer (yield: 7.4%).

The content of sulfur was obtained by an elemental analysis, and themolar ratio (MMD/PSVE) of the repeating unit based on MMD to therepeating unit based on PSVE in the polymer, and A_(R) were obtained,whereby MMD/PSVE=76.0/24.0, and A_(R)=0.82 meq/g. Further, the molecularweight of the polymer was measured by GPC, whereby the number averagemolecular weight as calculated as polymethyl methacrylate was 45,000,and the weight average molecular weight was 70,000.

Then, the obtained polymer was immersed in a solution ofKOH/H₂O/DMSO=11/59/30 (mass ratio) and maintained at 90° C. for 7 days.After cooling to room temperature, the polymer was washed with water andfurther immersed in water at 90° C. This washing with water was repeatedthree times. Further, it was immersed in 1N hydrochloric acid at 90° C.for one day, and after cooling to room temperature, it was washed withwater and further immersed in water at 90° C. This washing with waterwas repeated three times. Then, it was dried at 80° C. for 16 hours inan oven and further vacuum-dried at 80° C. to obtain an acid-formconverted MMD/PSVE-H copolymer.

In the same manner as in Preparation Example 1, an ethanol solutioncontaining 10 mass % of this polymer was prepared, a cast film wasprepared, and the softening temperature was measured and found to be135° C.

PREPARATION EXAMPLE 7 MMD/PSVE-H Copolymer 2

19.69 of a polymer was obtained (yield: 8.9%) in the same manner as inPreparation Example 6 except that PSVE charged was 207.19, MMD was 13.3g, nitrogen was introduced to bring the total pressure to 0.11 MPa(gauge pressure), and the reaction time was changed to 6 hours. In thesame manner as in Preparation Example 6, the molar ratio (MMD/PSVE) ofthe repeating unit based on MMD to the repeating unit based on PSVE inthe polymer, and A_(R), were obtained, whereby MMD/PSVE=66.7/33.3, andA_(R)=1.07 meq/g. Further, the number average molecular weight ascalculated as polymethyl methacrylate was 24,000, and the weight averagemolecular weight was 39,000.

The above polymer was acid-form converted in the same manner as inPreparation Example 6 to obtain a MMD/PSVE-H copolymer. In the samemanner as in Preparation Example 1, an ethanol solution containing 9.6mass % of this polymer was prepared, a cast film was prepared, and thesoftening point was measured and was found to be 125° C.

PREPARATION EXAMPLE 8 TFE/PDD/PSVE-H Copolymer

Into a 0.2 l autoclave, 14.39 of PDD, 52.6 g of PSVE, 76.9 g ofHCFC225cb and 0.36 g of IPP were put and freeze-deaerated. Afterintroducing 5.9 g of TFE, the temperature was raised to 40° C. toinitiate polymerization. The pressure at that time was 0.26 MPa (gaugepressure). The reaction was carried out at 40° C. for 10 hours, and whenthe pressure became 0.07 MPa (gauge pressure), the reaction wasterminated. The polymerization solution was put into hexane forprecipitation, followed by washing with hexane for three times. Vacuumdrying was carried out at 80° C. overnight to obtain 25.0 g of a polymer(yield: 34.4%).

By ¹⁹F-NMR, the molar ratio (TFE/PDD/PSVE) of the repeating unit basedon TFE, the repeating unit based on PDD and the repeating unit based onPSVE in the polymer was obtained, whereby TFE/PDD/PSVE=42/35/22, andA_(R) was 0.98 meq/g. Further, the number average molecular weight ascalculated as polymethyl methacrylate, by GPC, was 53,000 and the weightaverage molecular weight was 83,000.

Then, the above polymer was pressed at 160° C. to obtain a film having athickness of 100 μm. It was immersed in a solution ofKOH/H₂O/DMSO=11/59/30 (mass ratio) and maintained at 90° C. for 17 hoursfor hydrolysis. Then, after cooling to room temperature, it was washedwith water three times. Further, it was immersed in 2N sulfuric acid atroom temperature for 2 hours and then washed with water. This immersionin sulfuric acid and washing with water were carried out in a total ofthree times each, and finally washing with water was carried out threetimes. Drying at 80° C. for 16 hours in an oven was carried out, andfurther, vacuum drying at 80° C. was carried out to obtain a dry filmmade of an acid-form converted TFE/PDD/PSVE-H copolymer. The softeningtemperature was measured by the same method as in Preparation Example 1and was found to be 120° C. Further, the maximum stress in the tensiletest was 6.1 MPa and the elongation at breakage was 3.0%. It wasconfirmed that the film had a sufficient strength even when used as apolymer electrolyte membrane for a fuel cell.

Further, with respect to the above acid-form converted polymer, the sameoperation as in Preparation Example 1 was carried out to obtain a 14.5mass % ethanol solution.

The above tensile test of the film was carried out by cutting out thefilm in the shape of test specimen type 2 as stipulated in JIS K-7127(length: 150 mm, width: 10 mm, gauge length: 50 mm) and measuring undersuch conditions that the initial distance between chucks of 100 mm, atensile speed of 50 mm/min at 25° C. under a relative humidity of 50%.

EXAMPLE 1

A unit cell of Example 1 was prepared by the following procedure.Firstly, carbon having Pt supported thereon (amount of Pt supported: 54mass %) was dispersed in an ethanol solution containing 10 mass % of thePDD/PSVE-H copolymer to prepare a dispersion (mass of the carbon havingPt supported thereon: mass of the above copolymer=6:4). Then, thedispersion was sufficiently stirred and then further evaporated todryness to obtain a solid product, which was pulverized. Then, thispowder was re-dispersed in 2,2,3,3,3-pentafluoro-1-propanol to obtain acoating liquid for forming a catalyst layer of a cathode, wherein thesolid content concentration was 5 mass %.

Then, the carbon having Pt supported thereon (amount of Pt supported: 40mass %) was mixed and dispersed in ethanol and an ethanol solutioncontaining 9 mass % of the TFE/PSVE-H copolymer (A_(R)=1.1 meq/g), andwater was further added to obtain a coating solution for forming acatalyst layer of an anode wherein the solid content concentration was 8mass % (mass of ethanol: mass of water=1:1, mass of the carbon having Ptsupported thereon: mass of the above copolymer=7:3).

Further, as a gas diffusion layer for an anode and a cathode, a waterrepellent carbon powder layer (a mixture of carbon black and PTFE) wasloaded on one side of a water repellent carbon cloth (fiber wovenfabric) and further hot pressing was applied to obtain one having athickness of about 340 μm with the carbon powder layer surface to beattached to the catalyst layer formed, made flat. Further, as a polymerelectrolyte membrane, a polymer electrolyte membrane made of a sulfonicacid type perfluorocarbon polymer (FLEMION HR, trade name, manufacturedby Asahi Glass Company, Limited, A_(R)=1.1 meq/g, dry film thickness: 50μm) was prepared.

Then, a coating liquid for forming the catalyst layer of a cathode wascoated once on the water repellent carbon powder layer side of the abovegas diffusion layer, so that the amount of Pt would be 0.8 mg/cm²,followed by drying to form a catalyst layer, thereby to obtain acathode. On the other hand, in the same manner as the cathode, thecoating liquid for forming the catalyst layer of an anode was coatedonce on the water repellent carbon powder layer side of the above gasdiffusion layer sheet so that the amount of Pt would be 0.5 mg/cm²,followed by drying to form a catalyst layer thereby to obtain an anode.

Then, the obtained cathode and anode were cut out so that the effectiveelectrode area would be 25 cm². And, the cathode and the anode weredisposed so that the respective catalyst layers were located inside andfaced each other, and a polymer electrolyte membrane was interposedtherebetween, and hot pressing was carried out in that state to bond therespective catalyst layers of the cathode and anode with the polymerelectrolyte membrane to obtain a membrane electrode assembly.

COMPARATIVE EXAMPLE 1

A membrane electrode assembly was prepared in the same manner as inExample 1 except that both the anode and the cathode were prepared byusing the coating solution for forming the catalyst layer of an anode,prepared in Example 1.

Fuel Cell Performance Evaluation

To each of the membrane electrode assemblies of Example 1 andComparative Example 1, a separator made of carbon and having a gas flowpath formed was mounted to obtain a cell for measurement, and using anelectron load (FK400L, manufactured by Takasago Seisakusho K.K.) and adirect current power source (EX 750L, manufactured by TakasagoSeisakusho K.K.), a current voltage characteristic test of the cell formeasurement, was carried out. The measuring conditions were such thatthe hydrogen inlet pressure: 0.15 MPa, the air inlet pressure: 0.15 MPa,the operation temperature of the cell: 80° C., and the cell voltage (iRfree) was measured upon expiration of 10 hours after the operation at anoutput current density of 0.3 A/cm² and 1.0 A/cm², respectively.Further, the flow rates of the hydrogen gas and the air were adjusted sothat the hydrogen utilization rate would be 70%, and the air utilizationratio would be 40%, under the operation conditions. The results areshown in Table 1. TABLE 1 Cell characteristics Cell voltage upon Cellvoltage upon expiration of 10 hours expiration of 10 hours from theinitiation of the from the initiation operation (iR free)/mV at of theoperation 0.3 A · cm⁻² (iR free)/mV at 1 A · cm⁻² Example 1 820 725Comparative 770 680 Example 1

According to the present invention, a repeating unit having an alicyclicstructure is introduced in a copolymer for a solid polymer electrolytematerial, whereby it is possible to provide a solid polymer electrolytematerial having good ionic conductivity and water repellency and beingexcellent in gas permeability, a liquid composition containing such amaterial and a solid polymer fuel cell capable of providing a high celloutput constantly.

Further, the solid polymer electrolyte material of the present inventionhas a softening temperature higher than the conventional material, andthus has a feature that it can be used at a high temperature when it isused as an ion permselective membrane, a reverse osmosis membrane, afiltration membrane, a diaphragm, etc. in other electrochemicalprocesses as well as a solid polymer fuel cell.

The entire disclosure of Japanese Patent Application No. 2000-395511filed on Dec. 26, 2000 including specification, claims and summary areincorporated herein by reference in its entirety.

1: A liquid composition comprising an organic solvent having a hydroxylgroup in its molecule, and a solid polymer electrolyte material made ofa copolymer comprising a repeating unit based on a fluoromonomer A whichgives a polymer having an alicyclic structure in its main chain byradical polymerization, and a repeating unit based on a fluoromonomer B′of the following formula (1′):CF₂═CF(R^(f))_(j)SO₃M  (1′) wherein j is 0 or 1, M is a hydrogen atom,an alkali metal atom or a group of NR¹R²R³R⁴ (wherein each of R¹, R², R³and R⁴ which may be the same or different, is a hydrogen atom or amonovalent organic group), and R_(f) is a C₁₋₂₀ polyfluoroalkylene grouphaving a straight chain or branched structure which may contain etheroxygen atoms dissolved or dispersed in the organic solvent. 2: Theliquid composition according to claim 1, wherein the fluoromonomer B′ isrepresented by the following formula (2′), and the repeating unit basedon the fluoromonomer A is represented by any one of the followingformulae (3) to (5):CF₂═CFO(CF₂CFYO)_(k)(CF₂)_(m)SO₃M  (2′) wherein k is an integer of from0 to 2, m is an integer of from 1 to 12, Y is a fluorine atom or atrifluoromethyl group, and M has the same meaning as M in the aboveformula (1′);

wherein each of p, q and r which is independent of one another, is 0 or1, each of R^(f1) and R^(f2) which may be the same or different, is afluorine atom, a C₁₋₅ perfluoroalkyl group or a C₁₋₅ perfluoroalkoxygroup, and R^(f3) is a C₁₋₃ perfluoroalkylene group which may contain aC₁₋₅ perfluoroalkyl group or a C₁₋₅ perfluoroalkoxy group, as asubstituent;

wherein s is 0 or 1, each of R^(f4), R^(f5), R^(f6) and R^(f7) which maybe the same or different, is a fluorine atom or a C₁₋₅ perfluoroalkylgroup (provided that R^(f4) and R^(f5) may be connected to form a spiroring when s is 0), and R^(f8) is a fluorine atom, a C₁₋₅ perfluoroalkylgroup or a C₁₋₅ perfluoroalkoxy group; and

wherein each of R^(f9), R^(f10), R^(f11) and R^(f12) which may be thesame or different, is a fluorine atom or a C₁₋₅ perfluoroalkyl group. 3:The liquid composition according to claim 2, wherein the fluoromonomer Ais at least one member selected from the group consisting ofperfluoro(3-butenyl vinyl ether), perfluoro(2,2-dimethyl-1,3-dioxole),perfluoro(1,3-dioxole), 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxoleand perfluoro(2-methylene-4-methyl-1,3-dioxolane), and the fluoromonomerB′ is represented by the following formula (6′):CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃M  (6′) wherein k′ is 0 or 1, M has thesame meaning as M in the above formula (1′), and Y has the same meaningas Y in the above formula (2). 4: A solid polymer fuel cell comprisingan anode, a cathode and a polymer electrolyte membrane disposed betweenthe anode and the cathode, wherein the cathode contains, as aconstituting material, a solid polymer electrolyte material made of acopolymer comprising a repeating unit based on a fluoromonomer A whichgives a polymer having an alicyclic structure in its main chain byradical polymerization, and a repeating unit based on a fluoromonomer B′of the following formula (1″):CF₂═CF(R^(f))_(j)SO₃H  (1″) wherein j is 0 or 1, and R^(f) is a C₁₋₂₀polyfluoroalkylene group having a straight chain or branched structurewhich may contain ether oxygen atoms. 5: The solid polymer fuel cellaccording to claim 4, wherein the fluoromonomer B′ is represented by thefollowing formula (2″), and the repeating unit based on thefluoropolymer A is represented by any one of the following formulae (3)to (5):CF₂═CFO(CF₂CFYO)_(k)(CF₂)_(m)SO₃H  (2″) wherein k is an integer of from0 to 2, m is an integer of from 1 to 12, and Y is a fluorine atom or atrifluoromethyl group;

wherein each of p, q and r which is independent of one another, is 0 or1, each of R^(f1) and R^(f2) which may be the same or different, is afluorine atom, a C₁₋₅ perfluoroalkyl group or a C₁₋₅ perfluoroalkoxygroup, and R^(f3) is a C₁₋₃ perfluoroalkylene group which may contain aC₁₋₅ perfluoroalkyl group or a C₁₋₅ perfluoroalkoxy group, as asubstituent;

wherein s is 0 or 1, each of R^(f4), R^(f5), R^(f6) and R^(f7) which maybe the same or different, is a fluorine atom or a C₁₋₅ perfluoroalkylgroup (provided that R^(f4) and R^(f5) may be connected to form a spiroring when s is 0), and R^(f8) is a fluorine atom, a C₁₋₅ perfluoroalkylgroup or a C₁₋₅ perfluoroalkoxy group; and

wherein each of R^(f9), R^(f10), R^(f11) and R^(f12) which may be thesame or different, is a fluorine atom or a C₁₋₅ perfluoroalkyl group. 6:The solid polymer fuel cell according to claim 5, wherein thefluoromonomer A is at least one member selected from the groupconsisting of perfluoro(3-butenyl vinyl ether),perfluoro(2,2-dimethyl-1,3-dioxole), perfluoro(1,3-dioxole),2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole andperfluoro(2-methylene-4-methyl-1,3-dioxolane), and the fluoromonomer B′is represented by the following formula (6″):CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃H  (6″) wherein k′ is 0 or 1, and Y hasthe same meaning as Y in the above formula (2). 7: A fluoropolymer whichis a copolymer consisting essentially of a repeating unit of thefollowing formula (I) and a repeating unit based on a fluoromonomer D ofthe following formula (II), wherein the content of the repeating unitbased on the fluoromonomer D is from 10 to 75 mol %, and the numberaverage molecular weight is from 5,000 to 5,000,000:

CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃M  (II) wherein each of R^(f16) andR^(f17) which may be the same or different, is a fluorine atom or atrifluoromethyl group, k′ is 0 or 1, Y is a fluorine atom or atrifluoromethyl group, and M is a hydrogen atom, an alkali metal atom ora group of NR¹R²R³R⁴ (wherein each of R¹, R², R³ and R⁴ which may be thesame or different, is a hydrogen atom or a monovalent organic group). 8:A fluoropolymer which is a copolymer consisting essentially of arepeating unit based on perfluoro(3-butenyl vinyl ether) and a repeatingunit based on a fluoromonomer D of the following formula (II), whereinthe content of the repeating unit based on the fluoromonomer D is from10 to 75 mol %, and the number average molecular weight is from 5,000 to5,000,000:CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃M  (II) wherein k′ is 0 or 1, Y is afluorine atom or a trifluoromethyl group, and M is a hydrogen atom, analkali metal atom or a group of NR¹R²R³R⁴ (wherein each of R¹, R², R³and R⁴ which may be the same or different, is a hydrogen atom or amonovalent organic group). 9: A fluoropolymer which is a copolymerconsisting essentially of a repeating unit based onperfluoro(2-methylene-4-methyl-1,3-dioxolane) and a repeating unit basedon a fluoromonomer D of the following formula (II), wherein the contentof the repeating unit based on the fluoromonomer D is from 10 to 75 mol%, and the number average molecular weight is from 5,000 to 5,000,000:CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃M  (II) wherein k′ is 0 or 1, Y is afluorine atom or a trifluoromethyl group, and M is a hydrogen atom, analkali metal atom or a group of NR¹R²R³R⁴ (wherein each of R¹, R², R³and R⁴ which may be the same or different, is a hydrogen atom or amonovalent organic group). 10: A fluoropolymer which is a copolymerconsisting essentially of a repeating unit of the following formula (I),a repeating unit based on a fluoromonomer D of the following formula(II), and a repeating unit based on tetrafluoroethylene, wherein thecontent of the repeating unit of the following formula (I) is from 20 to60 mol %, the content of the repeating unit based on tetrafluoroethyleneis from 20 to 60 mol %, and the content of the repeating unit based onthe fluoromonomer D is from 10 to 40 mol %, and the number averagemolecular weight is from 5,000 to 5,000,000:

CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃M  (II) wherein each of R^(f16) andR^(f17) which may be the same or different, is a fluorine atom or atrifluoromethyl group, k′ is 0 or 1, Y is a fluorine atom or atrifluoromethyl group, and M is a hydrogen atom, an alkali metal atom ora group of NR¹R²R³R⁴ (wherein each of R¹, R², R³ and R⁴ which may be thesame or different, is a hydrogen atom or a monovalent organic group).11: A solid polymer electrolyte membrane which is a membrane made of apolymer electrolyte comprising a copolymer consisting essentially of arepeating unit of the following formula (I), a repeating unit based on afluoromonomer D of the following formula (II), and a repeating unitbased on tetrafluoroethylene, wherein the content of the repeating unitof the following formula (I) is from 20 to 60 mol %, the content of therepeating unit based on tetrafluoroethylene is from 20 to 60 mol %, andthe content of the repeating unit based on the fluoromonomer D is from10 to 40 mol %, and the number average molecular weight is from 5,000 to5,000,000:

CF₂═CFO(CF₂CFYO)_(k′)(CF₂)₂SO₃M  (II) wherein each of R^(f16) andR^(f17) which may be the same or different, is a fluorine atom or atrifluoromethyl group, k′ is 0 or 1, Y is a fluorine atom or atrifluoromethyl group, and M is a hydrogen atom, an alkali metal atom ora group of NR¹R²R³R⁴ (wherein each of R¹, R², R³ and R⁴ which may be thesame or different, is a hydrogen atom or a monovalent organic group).12: The fluoropolymer according to claim 10, which has an ion exchangecapacity of from 0.5 to 2.5 meq/g dry resin. 13: The fluoropolymeraccording to claim 10, wherein the copolymer has a softening temperatureof at least 100° C. 14: The solid polymer electrolyte membrane accordingto claim 11, which has an ion exchange capacity of from 0.5 to 2.5 meq/gdry resin. 15: The solid polymer electrolyte membrane according to claim11, wherein the copolymer has a softening temperature of at least 100°C.